Practical syntheses of penciclovir and famciclovir from N2-acetyl-7-benzylguanine

Article abstract of DOI:10.1016/j.tet.2006.03.080

We have established practical methods for the synthesis of penciclovir (PCV) and famciclovir (FCV) from readily available guanosine via N2-acetyl-7-benzylguanine. The alkylation of N2-acetyl-7-benzylguanine proceeded selectively at the N9 position to give the desired alkylated product in good yield in salt form. After conventional catalytic hydrogenolysis of the benzyl group and hydrolysis of the resulting acetate, pure PCV was obtained without the need for chromatography. As a side chain precursor, the mesylate was selected rather than a halide since the corresponding halides gave several impurities under the same reaction conditions. Two procedures for the synthesis of FCV from PCV and a derivative are also reported.

Full text of DOI:10.1016/j.tet.2006.03.080

Tetrahedron 62 (2006) 5709–5716
Practical syntheses of penciclovir and famciclovir from
N2-acetyl-7-benzylguanine
Takayoshi Torii,a Hiroshi Shiragami,a Keizo Yamashita,a Yumiko Suzuki,a Toyoto Hijiya,a
Tatsuki Kashiwagib and Kunisuke Izawaa,
*
aAminoScience Laboratories, Ajinomoto Co., Inc., 1-1, Suzuki-cho, Kawasaki-ku, Kawasaki 210-8681, Japan
bInstitute of Life Sciences, Ajinomoto Co., Inc., 1-1, Suzuki-cho, Kawasaki-ku, Kawasaki 210-8681, Japan
Received 28 February 2006; accepted 23 March 2006
Available online 12 April 2006
Abstract—We have established practical methods for the synthesis of penciclovir (PCV) and famciclovir (FCV) from readily available
guanosine via N2-acetyl-7-benzylguanine. The alkylation of N2-acetyl-7-benzylguanine proceeded selectively at the N9 position to give
the desired alkylated product in good yield in salt form. After conventional catalytic hydrogenolysis of the benzyl group and hydrolysis of
the resulting acetate, pure PCV was obtained without the need for chromatography. As a side chain precursor, the mesylate was selected rather
than a halide since the corresponding halides gave several impurities under the same reaction conditions. Two procedures for the synthesis of
FCV from PCV and a derivative are also reported.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
N7 ratio is less than 6:1.4a,b,g To improve this ratio, several
approaches have been reported, which involve changing
the C-6 substituent of 2-aminopurine.4h For example, Geen
et al. reported a ratio of 5.5:1 in the alkylation of 2-amino-
6-chloropurine and noted that it could be improved to 9:1
using 2-amino-6-iodopurine as the starting material.4c On
the other hand, this ratio was also improved by changing
the structure of the side chains.4d,e,k Recently, Toyokuni
et al. reported an excellent method using 2-phenyl-5-halo-
ethyl-1,3-dioxolane to give the N9-alkylated compound in
94% yield.4l However, in each case the regioselectivity was
not perfect, and it was difficult to eliminate N7 compounds
from the desired N9 product without column chromato-
graphy. When triethyl 3-bromopropane-1,1,1-tricarboxylate
was used as an alkyl side chain, the N7-alkylated compound
was easily separated by crystallization of the N9-alkylated
compound.4f Unfortunately, though, this reaction sequence
required longer steps than the case of using diacetoxy alkyl
halide side chain which is normally used to synthesize 1 and
2 from 2-amino-6-chloropurine. Geen et al. have further
developed several interesting methods, which involve the
coupling of 2-amino-6-chloropurine with an a-acetoxyfuran
derivative under acidic conditions4j and Pd-catalyzed N9-
selective allylation with an allyl acetate side chain.4i
However, these sequences again require multiple steps to
synthesize 1 and 2. It should be noted that 2-amino-6-chloro-
purine is not a desirable compound for use in large-scale
synthesis due to its high mutagenicity.5
Since Schaeffer et al. discovered that acyclovir is a potent
and selective anti-herpes virus agent, several groups have
undertaken intensive studies to develop still more potent and
effective acyclic nucleoside analogues.1 As a result, penci-
clovir (PCV) 1 and its pro-drug famciclovir (FCV) 2 were
found to be potent and highly selective anti-viral agents
against both the herpes simplex virus (HSV) and the vari-
cella-zoster virus (VZV).2 It has also been reported that 1
and 2 exhibit anti hepatitis B virus (HBV) activity.3 PCV 1
and FCV 2 are analogues of acyclovir that have alkyl side
chains at the N9 position (Fig. 1).
To synthesize 1 and 2, 2-amino-6-chloropurine is commonly
used as a starting material for coupling with alkyl halide side
chains.4 Normally, alkylation takes place at the N9 position
as well as at the N7 position of the purine moiety, and the N9/
O
N
N
N
HN
H2N
OAc
OAc
N
OH
OH
H2N
N
N
N
1
2
Figure 1. PCV 1 and FCV 2.
Keywords: Penciclovir; Famciclovir; Practical synthesis; N9-selective;
Alkylation.
Guanosine 3 is manufactured in very large scale by fermen-
tation. We have previously developed practical methods to
* Corresponding author. Tel.: +81 44 245 5066; fax: +81 44 244 9942;
e-mail: kunisuke_izawa@ajinomoto.com
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.03.080

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T. Torii et al. / Tetrahedron 62 (2006) 5709–5716
Ph
O
O
N
Ph
N
N
N
acid
HN
O
N
HN
H2N
hydrolysis
Br
BnBr
N
N
HN
H2N
N
H2N
HO
N
O
O
HO
HO
3
OH
HO
OH
Ph
O
N
O
Ph
side chain
R-Br
O
N
N
N
deprotection
HN
H2N
HN
H2N
N
Br
OH
OH
HN
OAc
N
N
N
H2N
N
OAc
1
Scheme 1. Synthesis of 7-benzylguanine and selective N9-alkylation.
synthesize first acyclovir by the chemical transpurination of
6a and then d4T via the enzymatic transglycosylation of 3.6b
respectively, in good yield (Scheme 2). We considered using
N-acetyl-20,30,50-tri-O-acetylguanosine12 as a starting mate-
rial for the synthesis of 7a, but discounted this idea because
deprotection of the N-acetyl group is inevitable in the degly-
cosylation step.
3
We considered that it may also be possible to use 3 as a start-
ing material for the synthesis of 1 and 2: in this respect, 3
may be considered an N9 ribofuranoside-protected guanine.
It is known that 3 can be converted to 7-benzylguanine in
two steps by conducting benzylation under neutral condi-
tions followed by acidic deglycosylation.7 We speculated
that if 7-benzylguanine could be coupled with the side
chains 8a–c at the N9 position under neutral conditions,
we might be able to obtain PCV 1 without the formation
of the N7-alkylated byproduct after reductive removal of
the N7 benzyl group (Scheme 1).8 We reported here a practi-
cal synthesis of PCV 1 using 7-benzylguanine derivatives
and also describe two methods for the synthesis of FCV 2
via 1 and its precursor.9
Ph
O
N
O
N
O
N
N
N
N
N
N
Ac2O
HN
H2N
HN
H2N
BnCl
NaBr
HN
AcONa
Br
H2N
AcO
N
O
O
AcOEt
DMF
O
HO
AcO
70 °C, 4 h
70 °C, 8 h
HO
3
OH
AcO
OAc
AcO
OAc
4
5
Ph
Ph
Ac2O, p-TsOH
O
O
conc.HCl
AcOH, 105 °C, 3 h
N
N
N
HN
H2N
HN
75% yield
(3 steps)
or BzCl, DMAP
2 HCl
N
pyridine, 96 °C, 2 h
N
RHN
N
6
7a : R = Ac (96%)
7b : R = Bz (97%)
2. Results and discussion
Scheme 2. Preparation of N2-protected 7-benzylguanines from guanosine 3.
7-Benzylguanine has previously been prepared by treating
guanosine 3 with benzyl bromide followed by acid hydroly-
sis.7 However, the reaction required dimethyl sulfoxide
(DMSO) as a solvent due to the low solubility of 3 in other
solvents. Since DMSO is not desirable for use in industrial-
scale production for reasons of cost and safety, we searched
for other reaction systems to prepare 7-benzylguanine.
When we used readily available 20,30,50-tri-O-acetylguano-
sine 4 as a starting material, the benzylation of 4 proceeded
well even in N,N-dimethylformamide (DMF) to give the de-
sired benzylated product as a bromide salt 5 in good yield. In
this reaction, the combination of benzyl chloride and sodium
bromide can be used instead of expensive benzyl bromide.
The benzylation reaction must be carried out below 70 ꢀC,
since the deglycosylation of 5 occurs at higher temperatures
to generate 7,9-dibenzylated guanine10 as a byproduct. After
the acid hydrolysis of 5, 7BnG$2HCl 6 was obtained in 75%
yield.11 In addition to the fact that a favorable solvent and
inexpensive benzyl chloride can be used, there is another
advantage in this reaction sequence, although the reaction
requires an additional acetylation step. Specifically, the free
form of 7-benzylguanine prepared by the previous method
was obtained as very low-density crystals, which were there-
fore difficult to filter off. On the other hand, upon formation
of the HCl salt, compound 6 could be handled more easily in
the isolation steps. To improve the solubility of 6 in the
reaction solvent, we carried out the acetylation and
benzoylation of 6 to give NAc7BnG 7a and NBz7BnG 7b,
First, we examined the coupling reactions of the 7BnG
derivatives 7a,b with the diacetate side chains 8a,b. In the
case of the coupling of NBz7BnG 7b with 8a4c, the coupling
product 9a could be obtained in crystalline form. After X-ray
crystal structural analysis and NMR studies, compound 9a
was unequivocally identified as the N9-alkylated compound
(Scheme 3). The coupling product 9a exists as a rather stable
ionic compound under neutral conditions, while acidic or ba-
sic hydrolysis produces decomposition products (Fig. 2).13
Ph
O
N
Ph
O
OAc
OAc DMF
N
N
HN
BzHN
I
N
N
+
HN
BzHN
OAc
OAc
I
85 °C
N
7b
8a
9a
Scheme 3. Preparation of an N9-alkylated 7-benzylguanine derivative.
However, when we used NAc7BnG 7a and side chains 8a,b,
the coupling products 9b,c could not be crystallized, and
therefore it was difficult to purify these products in this
step. Fortunately, after the debenzylation by catalytic hydro-
genolysis of 9b,c followed by alkaline hydrolysis of the ace-
tate, PCV 1 was easily isolated as crystals in respective
yields of 74 and 68%. This process did not require any
form of chromatographic purification to give pure PCV 1
in good yield (Scheme 4).

T. Torii et al. / Tetrahedron 62 (2006) 5709–5716
5711
reaction is that shown in Scheme 5. The coupling of 7a with
the bromide 8b gives 9c as a bromide salt. However, the bro-
mide anion of 9c attacks the benzyl carbon of 9c itself at a
higher temperature to give 10 and benzyl bromide. Ac3PCV
10 then reacts with extra 8b to give 11. NAc-Bn2G 12 must
arise by the reaction of benzyl bromide with remaining 7a.
We considered that it may be possible to suppress the side
reactions by changing the leaving group of the side chain
from halide to the less nucleophilic sulfonate. Thus, we tried
the coupling of NAc7BnG 7a with the mesylate 8c,4c which
is a precursor in the synthesis of iodide 8a and bromide 8b
and therefore less expensive. As we anticipated, the mesyl-
ate 8c was much less reactive than iodide 8a and bromide
8b. However, the reaction of NAc7BnG 7a with mesylate
8c was complete within 8 h in N-methylpyrrolidone (NMP)
when the reaction temperature was raised to 120 ꢀC. The re-
action proceeded without the formation of any impurities—
even at higher temperature. PCV 1 was synthesized by
coupling with NAc7BnG 7a and mesylate 8c, followed by
debenzylation and deacetylation in 76% without isolation
of any intermediates from NAc7BnG 7a. In addition to the
use of cheap 8c, there was another advantage in terms of
product purity, although the reaction yield was similar to
that using iodide 8a as a side chain.
Figure 2. X-ray crystal structure of the coupling product 9a.
Ph
O
N
Ph
6ClFCV 14 has been reported to be a key intermediate for
the synthesis of FCV 2.4f We investigated the transformation
of PCV 1 to 2. PCV 1 was converted to Ac2PCV 13 by selec-
tive acetylation of the hydroxy groups.15 Ac2PCV 13 was
then treated with phosphorus oxychloride, tetraethylammo-
nium chloride, and triethylamine at 80 ꢀC to give the known
intermediate 14 in 70% yield, in the same manner as has
been reported in the 6-chlorination of N-acetyl-20,30,50-tri-
O-acetyl-guanosine.16 6ClFCV 14 was transformed into
FCV 2 in 81% yield by the procedure described in the litera-
ture (Scheme 6).4f
O
OAc
OAc
N
N
HN
AcHN
X
N
N
+
HN
AcHN
OAc
OAc
NMP
X
N
7a
8a : X = I
9b : X = I
9c : X = Br
8b : X = Br
O
N
i) H2, Pd/C, K2CO3
ii) NaOH aq
HN
OH
OH
N
H2N
N
74% (from 8a)
68% (from 8b)
1
We also established another efficient synthetic route to
prepare FCV 2 from 10. Ac3PCV 10 was synthesized by
coupling with NAc7BnG 7a and 8c followed by catalytic
debenzylation at 50 ꢀC, in 78% yield based on 7a. Ac3PCV
10 was then converted to NAc6ClFCV 15 by chlorination in
the same manner as Ac2PCV 13. The reaction mixture of 15
was treated under acidic conditions in MeOH, where the
N-acetyl group of 15 was deprotected selectively to give
6ClFCV 14 (Scheme 7). The overall yield of 14 from
Ac3PCV 10 was 77%, which is better than the yield of 14
(64%) from PCV 1. Interestingly, the N-selective deacetyla-
tion of NAc6ClFCV 15 was accomplished under acidic
conditions.
Scheme 4. Preparation of PCV 1 from NAc7BnG 7a.
Next, we turned our attention to the comparative reactivities
with various alkyl side chains (see Table 1). In the case of the
bromide 8b,14 the yield of PCV 1 was lower than that of the
expensive iodide 8a (68 vs 74%) under the same reaction
conditions (80 ꢀC, 20 h). When the temperature in the reac-
tion of NAc7BnG 7a with 8b was raised, several byproducts
such as Ac3PCV 10, the 7,9-dialkylated compound 11, and
NAc-Bn2G 128b were detected by HPLC: clearly, the yield
of PCV 1 could not be improved simply by raising the
reaction temperature, and problematic impurities were also
generated. We speculate that the mechanism of the side
Table 1. N-Alkylation of NAc7BnG 7a
Entry
Side chain leaving group
Temp (ꢀC)
Time (h)
HPLC (area %)
9-Alkyl (9x)
NAc7BnG (7a)
Ac3PCV (10)
7,9-Alkyl (11)
NAc-Bn2G (12)
1
2
3
4
5
6
8a (I)
8b (Br)
8b (Br)
8c (OMs)
8c (OMs)
8c (OMs)
80
80
100
80
100
120
20
20
8
20
20
8
75.5
70.1
72.1
36.6
79.2
86.2
8.1
20.6
3.7
58.4
14.8
9.5
4.9
1.6
5.6
0.1
0.1
0.4
2.9
1.1
6.9
0.1
0.2
0.3
4.7
2
8.6
N.D.
N.D.
0.4

5712
T. Torii et al. / Tetrahedron 62 (2006) 5709–5716
O
N
OAc
OAc
N
N
HN
AcHN
Br
OAc
OAc
OAc
OAc
11
Br
Ph
8b
Ph
O
N
O
N
O
N
N
N
N
HN
AcHN
HN
AcHN
Br
N
N
OAc
OAc
HN
AcHN
OAc
OAc
N
7a
9c
10
Ph
Br
Ph
Br
O
N
N
HN
AcHN
N
Ph
12
Scheme 5. Possible mechanism for the N9-alkylation of NAc7BnG 7a.
Ph
O
H2, Pd/C
K2CO3
OAc
OAc
NaOH
N
+
HN
AcHN
NMP
CH3CN
50 °C, 5.5 h
H2O
60 °C, 1.5 h
MsO
120 °C, 8 h
N
N
7a
76% yield (3 steps)
8c
O
O
Ac2O
POCl3, Et4NCl
N
N
N
DMAP
PhNMe2
HN
H2N
HN
H2N
OH
OAc
OAc
DMF
CH3CN
80 °C, 1 h
N
N
N
45 °C, 1.5 h
OH
91% yield
70% yield
1
13
Cl
N
N
N
N
N
N
N
Geen et al.
OAc
OAc
OAc
OAc
H2N
N
81% yield
H2N
14
2
Scheme 6. Preparation of FCV 2 from NAc7BnG 7a.
O
Ph
O
H2, Pd/C
K2CO3
OAc
OAc
N
HN
N
+
OAc
OAc
HN
AcHN
NMP
CH3CN
50 °C, 5.5 h
78% yield
(2 steps)
N
AcHN
N
MsO
120 °C
N
N
8 h
7a
8c
10
Cl
Cl
N
POCl3, Et4NCl
NEt3
H+
N
N
N
OAc
OAc
OAc
OAc
CH3OH
15 °C, 25 h
CH3CN
80 °C, 1 h
N
N
AcHN
N
H2N
N
77% yield
(2 steps)
14
15
Scheme 7. Preparation of 6ClFCV 14 from NAc7BnG 7a.

T. Torii et al. / Tetrahedron 62 (2006) 5709–5716
5713
3. Conclusion
were added, and the mixture was stirred for 8 h at 70 ꢀC.
After the mixture was cooled to room temperature, concd
HCl (40.5 mL, 481 mmol) was added and the mixture was
stirred for 3 h at 40 ꢀC. After the mixture was cooled to
0 ꢀC, 8 M NaOH aq (42 mL, 336 mmol) and CH3OH
(50 mL) were added. The mixture was filtered off and
washed with 50% CH3OH aq (40 mL). After the residue
was dried, compound 6 (15.1 g, 75%) was obtained as color-
less crystals: mp 285 ꢀC (dec); IR (neat) 3406, 3122, 2732,
We have established practical synthetic routes to PCV 1 and
FCV 2. The yields of 1 and 2 were 76 and 49% from
NAc7BnG 7a, respectively; there was no need to purify
either by chromatography.
4. Experimental
4.1. General
1706, 1663, 1609, 1520, 1158, 843, 706, 679, 621 cmꢁ1
;
1H NMR (400 MHz, DMSO-d6) d 5.52 (2H, s, CH2), 7.32–
7.43 (5H, m, aryl), 8.34 (2H, br s, NH2), 8.91 (1H, s, H-8);
13C NMR (100 MHz, DMSO-d6) d 50.6, 107.5, 128.2,
128.6, 129.1, 136.2, 140.5, 150.9, 153.4, 154.5; MS
(FAB+) m/z 242 (7BnG-H+); HRMS (FAB+) calcd for
C12H12N5O (7BnG-H+): 242.1031, found, 242.1064; Anal.
Calcd for C12H13Cl2N5O: C, 45.88; H, 4.17; N, 22.29.
Found: C, 46.17; H, 4.30; N, 22.69.
All reagents were purchased and used without further purifi-
cation. Thin-layer chromatography (TLC) was conducted on
precoated TLC plates (Merck 60F254). High-performance
liquid chromatography (HPLC) was performed with a Hita-
chi L-6000 pump and L-4000 UV detector system using a
YMC-ODS-A column. Melting points were measured with
a Buchi B-545. NMR spectra were obtained on a BRUKER
¨
Advance-400 MHz spectrometer. All 1H NMR spectra were
measured in DMSO-d6 solvent, and chemical shifts are re-
ported as d values in parts per million relative to tetramethyl-
silane (d 0.00) as an internal standard. All 13C NMR spectra
were measured in DMSO-d6 solvent, and chemical shifts are
reported as d values in parts per million relative to DMSO-d6
(d 39.5) as an internal standard. Infrared (IR) spectra were
recorded on an ASI React IR 1000 FTIR spectrometer
with an ATR sampling system and are reported in wave num-
ber (cmꢁ1). Mass spectra (MS) and High-resolution mass
spectra (HRMS) were obtained with a JEOL JMS-700V
(JEOL Datum Ltd).
4.1.3. N2-Acetyl-7-benzylguanine (7a). A mixture of com-
pound 6 (4.37 g, 13.9 mmol) in AcOH (9 mL) was added to
acetic anhydride (4.44 mL, 45.1 mmol) and p-TsOH$H2O
(132.8 mg, 0.70 mmol) and the resulting solution was stirred
for 3 h at 105 ꢀC. After the mixture was cooled to room tem-
perature, water (180 mL), 5% NaHCO3 aq (150 mL) and
1 M NaOH aq (100 mL) were added to give pH 5.3. The
slurry was filtered off and washed with water (50 mL). After
the residue was dried, NAc7BnG 7a (3.77 g, 95.7%) was ob-
tained as colorless crystals: mp 237–238 ꢀC (lit.18 241 ꢀC);
IR (neat) 3141, 1713, 1686, 1619, 1546, 1424, 1364, 1246,
1
1216, 778, 710, 652 cmꢁ1; H NMR (400 MHz, DMSO-
d6) d 2.15 (3H, s, NAc), 5.51 (2H, s, CH2), 7.27–7.36 (5H,
m, aryl), 8.34 (1H, s, H-8), 11.56 (1H, br s, NH); 13C
NMR (100 MHz, DMSO-d6) d 20.5, 49.6, 111.5, 127.9,
128.2, 129.0, 137.7, 144.7, 147.4, 153.0, 157.6, 173.7; MS
(FAB+) m/z 284 (MH+); HRMS (FAB+) calcd for
C14H14N5O2 (MH+): 284.1148, found, 284.1147.
4.1.1. 20,30,50-Tri-O-acetylguanosine (4). A mixture of gua-
nosine 3 sodium salt (66.58 g, 0.20 mol) in AcOEt (400 mL)
was added to acetic anhydride (79.3 mL, 0.84 mol) and
stirred for 2 h at 60 ꢀC. After the mixture was cooled to
room temperature, water (200 mL) was added, and the mix-
ture was separated. The organic layer was concentrated in
vacuo. To the residue was added water (1.0 L) and 1 M
NaOH aq to pH 5.0. The mixture was filtered off and washed
with water (1.0 L). After the residue was dried, compound 4
(74.4 g, 91%) was obtained as colorless crystals: mp 224–
227 ꢀC (lit.17 225–227 ꢀC); IR (neat) 3149, 2738, 1740,
4.1.4. N2-Benzoyl-7-benzylguanine (7b). A mixture of
compound 6 (7.29 g, 23.2 mmol) in pyridine (47 mL) was
added to 4-dimethylaminopyridine (145.6 mg, 1.19 mmol).
Benzoyl chloride (5.5 mL, 47.4 mmol) was added dropwise
and the mixture was stirred for 2 h at 96 ꢀC. After the mix-
ture was cooled to room temperature, AcOEt (95 mL) was
added and the mixture was stirred for 0.5 h at room tempera-
ture. The precipitated crystals were collected on filter and
washed with AcOEt. The crystals were added to water
(95 mL) and stirred for 2 h at room temperature. The slurry
was filtered off and washed with water. After the residue was
dried, NBz7BnG 7b (7.79 g, 97.0%) was obtained as an off-
white powder: mp 265–267 ꢀC; IR (neat) 3089, 1684, 1657,
1609, 1534, 1370, 1268, 1233, 903, 782, 697, 658 cmꢁ1; 1H
NMR (400 MHz, DMSO-d6) d 5.55 (2H, s, CH2), 7.28–7.39
(5H, m, aryl), 7.53–7.58 (2H, m, aryl), 7.64–7.69 (1H, m,
aryl), 8.03–8.07 (2H, m, aryl), 8.39 (1H, s, H-8), 11.86
(1H, br s, NH); 13C NMR (100 MHz, DMSO-d6) d 49.6,
111.9, 127.9, 128.3, 128.7, 128.9, 129.1, 132.6, 133.4,
137.7, 144.8, 147.5, 153.2, 157.6, 169.1; MS (FAB+) m/z
346 (MH+); HRMS (FAB+) calcd for C19H16N5O2 (MH+):
346.1304, found, 346.1315.
1686, 1212, 1073, 783, 675, 635 cmꢁ1
;
1H NMR
(400 MHz, DMSO-d6) d 2.04 (3H, s, OAc), 2.05 (3H, s,
OAc), 2.11 (3H, s, NAc), 4.26 (1H, dd, J¼5.6, 11.2 Hz,
H50-a), 4.29–4.34 (1H, m, H-40), 4.38 (1H, dd, J¼3.6,
11.2 Hz, H-50-b), 5.49 (1H, dd, J¼4.1, 5.9 Hz, H-30), 5.79
(1H, t, J¼6.0 Hz, H-20), 5.98 (1H, d, J¼6.1 Hz, H-10), 6.53
(2H, br s, NH2), 7.93 (1H, s, H-8), 10.72 (1H, br s, NH);
13C NMR (100 MHz, DMSO-d6) d 20.5, 20.7, 20.9, 63.4,
70.7, 72.4, 79.9, 84.8, 117.2, 136.0, 151.5, 154.3, 157.0,
169.6, 169.8, 170.4; MS (FAB+) m/z 410 (MH+), 432
(M+Na+); HRMS (FAB+) calcd for C16H20N5O8 (MH+):
410.1312, found, 410.1310.
4.1.2. 7-Benzylguanine dihydrochloride (6).11 A solution
of guanosine 3 (13.9 g, 48.1 mmol) in DMF (28.3 mL) was
added to sodium acetate (985.6 mg, 12.0 mmol) and acetic
anhydride (19.1 mL, 202 mol) and the mixture was stirred
for 4 h at 70 ꢀC. After the mixture was cooled to 30 ꢀC,
concd HCl (19.1 mL, 202 mmol), sodium bromide (8.89 g,
86.5 mmol), and benzyl chloride (6.64 mL, 57.7 mmol)
4.1.5. 9-(4-Acetoxy-3-acetoxymethylbut-1-yl)-N2-benzoyl-
7-benzylguaninium iodide (9a). A mixture of NBz7BnG 7b

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T. Torii et al. / Tetrahedron 62 (2006) 5709–5716
(3.579 g, 10.0 mmol) in DMF (7.5 mL) was added to iodide
8a (3.25 g, 10.0 mmol), and the mixture was stirred for 18 h
at 85 ꢀC. After the mixture was cooled to 75 ꢀC, AcOEt
(30 mL) was added and the mixture was cooled to 0 ꢀC. The
mixture was filtered off and washed with AcOEt (15 mL).
After the residue was dried, compound 9a (4.41 g, 64%)
was obtained as a pale yellowish powder: mp 180–181 ꢀC;
IR (neat) 3320, 3160, 2977, 1711, 1596, 1420, 1241, 787,
pH over 13.0 and the mixture was stirred for 2 h at 50 ꢀC.
After the mixture was cooled to room temperature, the pH
of the solution was adjusted to 7.0 by 6 M HCl aq and the
solution was cooled to 0 ꢀC. The mixture was filtered off and
washed with cold water (5 mL). After the residue was dried,
PCV 1 (2.58 g, 68%) was obtained as colorless crystals.
4.1.8. Penciclovir (1) from mesylate 8c. A mixture of
NAc7BnG 7a (1.67 g, 5.84 mmol) in 1-methylpyrrolidone
(1.2 mL) was added to mesylate 8c (1.68 g, 5.84 mmol)
and this mixture was stirred for 8 h at 120 ꢀC. After the mix-
ture was cooled to room temperature, CH3CN (29.2 mL), 5%
Pd–C (50% wet, 0.515 g), and K2CO3 (0.485 g, 3.50 mmol)
were added and the mixture was stirred at 50 ꢀC under a hy-
drogen atmosphere (1 atm) for 10 h. After the mixture was
cooled to room temperature, 4 M NaOH aq (4.4 mL) was
added and filtered off, the residue was washed with 2 M
NaOH aq (2ꢂ3 mL), and the filtrate was concentrated. To
the residue was added 2 M NaOH aq (8.5 L) and the mixture
was stirred for 1.5 h at 60 ꢀC. After the mixture was cooled
to room temperature, the solution was washed with CH2Cl2
(10 mL), and the pH of the water layer was adjusted to 6.7 by
concd H2SO4. The mixture was stirred for 1 h at room tem-
perature and filtered off, and the residue was washed with
cold water (3 mL). After the residue was dried, PCV 1
(1.12 g, 76%) was obtained as colorless crystals.
710 cmꢁ1 1H NMR (400 MHz, DMSO-d6) d 1.96–2.04
;
(2H, m, H-20), 2.02 (6H, s, Acꢂ2), 2.06–2.12 (1H, m, H-
30), 4.06 (4H, d, J¼5.7 Hz, H-40), 4.37 (2H, t, J¼7.4 Hz,
H-10), 5.73 (2H, s, CH2), 7.38–7.47 (3H, m, aryl), 7.50–
7.53 (2H, m, aryl), 7.56–7.61 (2H, m, aryl), 7.69–7.74
(1H, m, aryl), 8.03–8.06 (2H, m, aryl), 9.74 (1H, s, H-8);
13C NMR (100 MHz, DMSO-d6) d 21.0, 27.8, 34.5, 44.1,
52.0, 63.7, 111.1, 128.7, 129.0, 129.0, 129.2, 129.3, 132.1,
134.0, 134.6, 140.4, 148.0, 151.3, 152.1, 169.5, 170.7; MS
(FAB+) m/z 532 (MꢁI+); HRMS (FAB+) calcd for
C28H29N5O6 (MꢁI+): 532.2191, found, 532.2214.
4.1.6. Penciclovir (1) from iodide 8a. A mixture of
NAc7BnG 7a (20.6 g, 70.5 mmol) in 1-methylpyrrolidone
(23.5 mL) was added to iodide 8a (22.9 g, 70.5 mmol),
and the mixture was stirred for16 h at 80 ꢀC. After the mix-
ture was cooled to room temperature, 50% CH3OH aq
(236 mL), 5% Pd–C (50% wet, 6.34 g), and K2CO3
(14.9 g, 141 mmol) were added and the mixture was stirred
at 45 ꢀC under a hydrogen atmosphere (1 atm) for 21.5 h.
After the mixture was cooled to room temperature, 4 M
NaOH aq (35.5 mL) was added and filtered off, the residue
was washed with 2 M NaOH aq (60 mL), and the filtrate was
concentrated. To the residue was added water (20 mL), and
this mixture was stirred for 1 h at 60 ꢀC. After the mixture
was cooled to room temperature, the pH of the solution
was adjusted to 11.5 by 2 M HCl aq and the precipitated
7BnG was filtered off. The pH of the filtrate was adjusted
to 6.0 by 6 M HCl aq, and then the temperature was lowered
to 0 ꢀC. The mixture was filtered off and washed with cold
water (30 mL). After the residue was dried, PCV 1 (13.4 g,
74%) was obtained as colorless crystals: mp 270–272 ꢀC
(lit.4b 275–277 ꢀC); IR (neat) 3411, 3128, 2881, 2667,
4.1.9. 9-(4-Acetoxy-3-acetoxymethylbut-1-yl)-N2-acetyl-
guanine (10). A mixture of NAc7BnG 7a (2.86 g,
10.0 mmol) in 1-methylpyrrolidone (2.0 mL) was added to
8c (2.87 g, 10.0 mmol) and the mixture was stirred for 8 h
at 120 ꢀC. After the mixture was cooled to 50 ꢀC, CH3CN
(50 mL), 5% Pd–C (50% wet, 0.88 g), and K2CO3 (0.83 g,
6.02 mmol) were added and the mixture was stirred at
50 ꢀC under a hydrogen atmosphere (1 atm) for 5.5 h.
AcOH (20.0 mL) was added and filtered off at 50 ꢀC, and
the residue was washed with AcOH (10 mL). After the mix-
ture was cooled to room temperature, the filtrate was concen-
trated in vacuo, and coevaporated with toluene (10 mL). To
the residue was added AcOEt (20 mL), and the mixture was
stirred for 1 h at room temperature, filtered off, washed with
AcOEt (10 mL). The wet crystals were added to water
(40 mL), and then stirred for 0.5 h at 60 ꢀC and for 2 h at
room temperature. The mixture was filtered off and washed
with water. After the mixture was dried, Ac3PCV 10 (3.03 g,
78%) was obtained as a pale yellowish powder: mp 226–
227 ꢀC (lit.8c 222–223 ꢀC); IR (neat) 3267, 3085, 1729,
1
1675, 1602, 1397, 1196, 1054, 992, 781, 753 cmꢁ1; H
NMR (400 MHz, DMSO-d6) d 1.41–1.48 (1H, m, H-30),
1.67–1.74 (2H, m, H-20), 3.32–3.38 (2H, m, H-40-a), 3.39–
3.46 (2H, m, H-40-b), 4.00 (2H, t, J¼7.4 Hz, H-10), 4.41
(2H, t, J¼5.2 Hz, OHꢂ2), 6.41 (2H, br s, NH2), 7.68 (1H,
s, H-8), 10.50 (1H, br s, NH); 13C NMR (100 MHz,
DMSO-d6) d 29.2, 41.1, 41.4, 61.7, 116.9, 137.7, 151.5,
153.8, 157.2; MS (FAB+) m/z 254 (MH+); HRMS (FAB+)
calcd for C10H16N5O3 (MH+): 254.1253, found, 254.1230.
1
1663, 1603, 1540, 1262, 1233, 785, 739, 633 cmꢁ1; H
NMR (400 MHz, DMSO-d6) d 1.82–1.88 (2H, m, H-20),
1.90–1.97 (1H, m, H-30), 2.00 (6H, s, OAcꢂ2), 2.18 (3H,
s, NAc), 4.02 (4H, d, J¼5.3 Hz, H-40), 4.15 (2H, t,
J¼7.1 Hz, H-10), 8.02 (1H, s, H-8), 11.66 (1H, br s, NH);
13C NMR (100 MHz, DMSO-d6) d 21.0, 24.2, 28.6, 34.8,
41.4, 63.8, 120.5, 140.1, 148.0, 148.9, 155.3, 170.7, 173.8;
MS (FAB+) m/z 380 (MH+); HRMS (FAB+) calcd for
C16H22N5O6 (MH+): 380.1570, found, 380.1556.
4.1.7. Penciclovir (1) from bromide 8b. A mixture of
NAc7BnG 7a (4.44 g, 15.0 mmol) in 1-methylpyrrolidone
(5.0 mL) was added to bromide 8b (4.23 g, 15.0 mmol)
and this mixture was stirred for 18 h at 80 ꢀC. After the mix-
ture was cooled to room temperature, 50% CH3OH aq
(60 mL), 5% Pd–C (50% wet, 1.37 g), and K2CO3 (3.18 g,
30.0 mmol) were added and the mixture was stirred at
45 ꢀC under a hydrogen atmosphere (1 atm) for 21.5 h. After
the mixture was cooled to room temperature, 25% NaOH aq
(4.85 g) was added and filtered off, the residue was washed
with 50% CH3OH aq (20 mL), and the filtrate was concen-
trated. To the residue was added 25% NaOH aq (1.18 g) to
4.1.10. 9-(4-Acetoxy-3-acetoxymethylbut-1-yl)guanine
(13). A mixture of PCV 1 (12.5 g, 48.8 mmol) in DMF
(46 mL) was added to 4-dimethylaminopyridine (0.3 g,
2.5 mmol) and acetic anhydride (10.6 mL, 112.2 mmol),
and the mixture was stirred for 1.5 h at 45 ꢀC. After the mix-
ture was cooled to room temperature, 2-propanol (115 mL)

T. Torii et al. / Tetrahedron 62 (2006) 5709–5716
5715
was added and the mixture was stirred for 1.5 h at 0 ꢀC. The
mixture was filtered off and washed with 2-propanol
(2ꢂ15 mL). The wet crystals were added to AcOEt
(105 mL), warmed to 70 ꢀC, and cooled to room tempera-
ture. The mixture was filtered off and washed with AcOEt
(2ꢂ15 mL). After the residue was dried, Ac2PCV 13
(15.3 g, 91%) was obtained as colorless crystals: mp 198–
202 ꢀC (lit.15a 202–205 ꢀC); IR (neat) 3323, 3159, 2730,
satd NaHCO3 aq, and the mixture was stirred for 1.5 h at
0 ꢀC. The mixture was filtered off and washed with water.
After the residue was dried, 6ClFCV 14 (1.42 g, 77%) was
obtained as a pale yellowish powder.
5. Crystallographic data
1735, 1686, 1605, 1366, 1221, 1036, 785, 691, 639 cmꢁ1
;
Crystallographic data for the structure reported in this
paper have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication number
CCDC600088. Copies of the data can be obtained, free of
charge, on application on CCDC, 12 Union Road, Cam-
bridge CB2 1EZ, UK [fax: +44 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk].
1H NMR (400 MHz, DMSO-d6) d 1.77–1.84 (2H, m, H-20),
1.88–1.95 (1H, m, H-30), 2.00 (6H, s, Acꢂ2), 4.00–4.04
(6H, m, H-10 and H-40), 6.40 (2H, br s, NH2), 7.70 (1H, s,
H-8), 10.52 (1H, br s, NH); 13C NMR (100 MHz, DMSO-
d6) d 20.5, 28.1, 34.3, 40.3, 63.3, 116.5, 137.2, 151.0,
153.4, 156.7, 170.2; MS (FAB+) m/z 338 (MH+); HRMS
(FAB+) calcd for C14H20N5O5 (MH+): 338.1464, found,
338.1462.
References and notes
4.1.11. 9-(4-Acetoxy-3-acetoxymethylbut-1-yl)-2-amino-
6-chloropurine (14), from Ac2PCV 13. A mixture of
Ac2PCV 13 (5.16 g, 15.0 mmol) and tetraethylammonium
chloride (4.97 g, 30.0 mmol) in CH3CN (30 mL) was cooled
to 0 ꢀC and added to N,N-dimethylaniline (1.0 mL, 7.5 mmol)
and phosphorus oxychloride (6.25 mL, 67.5 mmol). The
mixture was stirred for 1 h at 70 ꢀC and evaporated. The resi-
due was diluted with CH2Cl2 (60 mL) and washed with satd
NaHCO3 aq (2ꢂ45 mL) and water (30 mL). The CH2Cl2
layer was evaporated in vacuo. To the residue was added
CH3OH (7 mL) and H2O (3.5 mL) and the mixture was
stirred for 1.5 h at ꢁ5 ꢀC. The mixture was filtered off and
washed with 75% CH3OH aq. After the residue was dried,
the crude solid of 6ClFCV 14 (3.96 g) was obtained as an
off-white solid. Recrystallization from 2-propanol gave
pure 6ClFCV 14 (3.74 g, 70%) as a pale yellowish powder:
mp 132–134 ꢀC (lit.4c 134–136 ꢀC); IR (neat) 3485, 3301,
3195, 1744, 1729, 1623, 1559, 1472, 1239, 1023, 907,
1. (a) Schaeffer, H. J.; Beauchamp, L.; De Miranda, P.; Elion,
G. B.; Bauer, D. J.; Collins, P. Nature 1978, 272, 583–585;
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2. (a) Jarvest, R. L.; Harnden, M. R. EP Patent 141927, 1984; (b)
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880, 647 cmꢁ1; H NMR (400 MHz, DMSO-d6) d 1.83–
4. (a) Pandit, U. K.; Grose, W. F. A.; Eggelte, T. A. Synth. Com-
mun. 1972, 2, 345–351; (b) Harnden, M. R.; Jarvest, R. L. Tet-
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(h) Brand, B.; Reese, C. B.; Song, Q.; Visintin, C. Tetrahedron
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Share, A. C.; Slater, G. R.; Smith, N. M. Tetrahedron 2000, 56,
4589–4595; (j) Geen, G. R.; Kincey, P. M.; Spoors, P. G. Tetra-
hedron Lett. 2001, 42, 1781–1784; (k) Kim, D.-K.; Kim, Y.-W.;
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5. Kaina, B.; Aurich, O. Mutat. Res. 1984, 139, 149–154. Muta-
genic properties of 2-amino-6-chloropurine were observed
by micronucleus and Ames tests, though NAc7BnG 7a was
negative in Ames test. These tests were performed by Japan
Biological Science Center Inc.
1.89 (2H, m, H-20), 1.90–1.95 (1H, m, H-30), 1.99 (6H, s,
Acꢂ2), 4.01 (4H, d, J¼5.6 Hz, H-40), 4.13 (2H, t,
J¼7.1 Hz, H-10), 6.90 (2H, br s, NH2), 8.16 (1H, s, H-8);
13C NMR (100 MHz, DMSO-d6) d 20.9, 28.1, 34.8, 41.2,
63.8, 123.7, 143.5, 149.7, 154.4, 160.1, 170.7; MS (FAB+)
m/z 356 (MH+); HRMS (FAB+) calcd for C14H19ClN5O4
(MH+): 356.1126, found, 356.1131.
4.1.12. 9-(4-Acetoxy-3-acetoxymethylbut-1-yl)-2-amino-
6-chloropurine (14), from Ac3PCV 10. A mixture of
Ac3PCV 10 (1.93 g, 5.02 mmol) and tetraethylammonium
chloride (1.66 g, 10.4 mmol) in CH3CN (10 mL) was cooled
to 0 ꢀC, and triethylamine (0.35 mL, 2.51 mmol) and phos-
phorus oxychloride (2.09 mL, 22.6 mmol) were added.
The mixture was stirred for 1 h at 80 ꢀC and evaporated.
The residue was diluted by CH2Cl2 (9 mL). After the mix-
ture was cooled to 0 ꢀC, 2 M NaOH aq (6 mL) and 4 M
NaOH aq (3 mL) were added dropwise, and the mixture
was separated. To the CH2Cl2 layer was added satd NaHCO3
aq (4 mL) and water (2 mL), and the mixture was stirred for
0.5 h at room temperature. After layer separation, the
CH2Cl2 layer was evaporated in vacuo. To the residue was
added CH3OH (6 mL), and the mixture was stirred 25 h at
15 ꢀC, while the progression of deacetylation was con-
firmed. At that time the pH of the solution was 0.6. After
deacetylation, the pH of the mixture was adjusted to 7.0 by

5716
T. Torii et al. / Tetrahedron 62 (2006) 5709–5716
6. (a) Shiragami, H.; Ineyama, T.; Uchida, Y.; Izawa, K. Nucleo-
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K. Nucleosides Nucleotides 1995, 14, 337–340.
5.83 (1H, t, J¼7.5 Hz, NH), 6.69 (2H, br s, NH2), 7.13–7.70
(5H, m, aryl), 7.84 (1H, s, CHO); MS (ESI+) m/z 362 (MH+).
See: Hecht, S. M.; Adams, B. L.; Kozarich, J. W. J. Org.
Chem. 1976, 41, 2303–2311.
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Ph
O
NCHO
HN
OH
NH
H2N
N
OH
9. We had already reported the preliminary results of this study.
See: Izawa, K.; Shiragami, H. Pure Appl. Chem. 1998, 70,
313–318.
¨
14. Lindborg, B. G.; Datema, R.; Johansson, K. N. G.; Oeberg,
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´
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13. We obtained the decomposed compound by basic hydrolysis.
The compound was identified as depicted below. 1H NMR
(400 MHz, DMSO-d6) d 1.22–1.29 (2H, m, H-20), 1.38–1.44
(1H, m, H-30), 3.08–3.50 (6H, m, H-10 and H-40), 4.42 (1H,
d, J¼19.0 Hz, PhCH2-a), 4.55 (1H, d, J¼19.0 Hz, PhCH2-b),
2607; (b) Muraoka, M.; Iwahara, J.; Kaneko, M. Chem. Pharm.
Bull. 1986, 34, 2609–2613; (c) Kozai, S.; Yorikane, A.;
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